Melatonin is a neurohormone produced by the pineal glands in the brain, mainly at night.

It prepares the body for sleep and is sometimes called the “hormone of sleep” or “hormone of darkness.”

Melatonin supplements are frequently used as a sleep aid. They help you fall asleep, improve sleep quality and increase sleep duration. However, they don’t appear to be as effective as many other sleep medications ().

Sleep isn’t the only body function affected by melatonin. This hormone also plays a role in the body’s antioxidant defenses and helps regulate blood pressure, body temperature and cortisol levels, as well as sexual and immune function ().

In the US, melatonin is available over-the-counter. In contrast, it is a prescription drug in Australia and most European countries and only approved for use in older adults with sleep disorders (, ).

Its use is growing, raising concerns about its possible side effects.

Summary Melatonin is a hormone produced by the brain in response to fading light. It prepares the body for sleep and is often used as a sleep aid.

A few studies have investigated the safety of melatonin, but none have revealed any serious side effects. It also doesn’t seem to cause any dependence or withdrawal symptoms (, ).

Nonetheless, some medical practitioners are concerned that it may reduce the natural production of melatonin in the body, but short-term studies suggest no such effects (, , ).

Several studies have reported general symptoms, including dizziness, headache, nausea or agitation. However, these were equally common in the treatment and placebo groups and couldn’t be attributed to melatonin ().

Melatonin supplements are generally considered safe in the short-term, even when taken in very high doses. However, more research on its long-term safety is needed, especially in children ().

A few mild side effects and drug interactions are discussed in the chapters below.

Summary Melatonin supplements are considered safe, and no studies have revealed any serious side effects to date. Yet, more research is needed to evaluate its long-term effects.

When taken at other times of the day, they may cause undesirable sleepiness. Keep in mind that sleepiness is technically not a side effect but rather their intended function (, ).

Nevertheless, sleepiness is a possible problem in people who have reduced melatonin clearance rates, which is the rate at which a drug is removed from the body. An impaired clearance rate extends the time melatonin levels stay high after taking supplements.

While this may not be an issue in most healthy adults, reduced melatonin clearance has been reported in older adults and infants. It is unknown whether this has any effects on melatonin levels the morning after taking supplements (, ).

Yet, even when melatonin supplements or injections are given during the daytime, they do not seem to affect the ability to maintain focus.

Studies in healthy people injected with 10 or 100 mg of melatonin or given 5 mg by mouth found no effects on reaction times, attention, concentration or driving performance, compared to a placebo (, ).

More studies are needed before scientists can fully understand the effects of melatonin supplements on daytime sleepiness.

Summary Melatonin supplements may cause daytime sleepiness when taken during the day. You should only use melatonin in the evening.

To aid sleep, the standard dosage ranges from 1 to 10 milligrams per day. However, the optimal dose has not been formally established ().

Since not all melatonin supplements are the same, make sure to follow the instructions on the label.

Also, keep in mind that the quality of over-the-counter supplements is not monitored by health authorities. Try to choose brands that are reputable and certified by a third party, such as Informed Choice and NSF International.

Many experts do not recommend their use in children and adolescents until more evidence confirms its safety in these groups ().

Since melatonin is transferred into breast milk, breastfeeding mothers should keep in mind that it might cause excessive daytime sleepiness in nursing infants ().

Summary The common dosage of melatonin ranges from 1–10 mg per day, but make sure to follow the instructions on the label. Parents should not give it to their children without first consulting their medical provider.